1. Field of the Invention
The present invention relates to a method and apparatus for burning waste materials such as household garbage or industrial waste, utilizing a rotary tubular kiln and combustion air which has been preheated in a special manner to provide extremely high combustion air temperatures.
2. Description of the Prior Art
The economical elimination of waste materials such as household garbage and industrial wastes presents a difficult problem. This is particularly true when such waste materials are eliminated in a standard incinerating system. Such systems are predominantly equipped with grate furnaces which must be operated with substantial excesses of air in order to approximate complete combustion. For example, excess air factors of 1.8 to 2.2 are standard in such grate furnaces. Considerable disadvantages occur as a result, among other things, because the furnace operated in such a way generates great quantities of flue gas at a relatively low temperature, for example, between 800.degree. and 900.degree. C. The efficiency of any means for energy recovery inserted into such a stream of flue gas, for example, for generating steam or power, is consequently relatively low. In addition, great quantities of flue gas require large and consequently expensive exhaust gas scrubbing systems.
As a result of the non-homogeneity as well as differences in calorific values of the wastes, and the difficulty of agitating these materials in a grate type incinerator to bring them in contact with combustion air, zones exist in the incinerator in which the required temperature of 800.degree. C. is not reached. In addition, there may be an inadequate supply of oxygen in some zones. This situation is particularly critical when relatively damp waste material or waste material low in calorific value is employed. Uncontrollable regions arise in the incinerator in which combustion at temperatures below 800.degree. C. occurs. At these temperature ranges, however, polychlorinated dibenzodioxins and dibenzofuranes, particularly TCDD and TCDF can be formed in the combustion process in the presence of chlorine and aromatic hydrocarbon molecules such as exist, for example, in chlorine-containing plastics, lignosulphonic acids, pentachlorophenol, polychlorinated biphenyls and the like. The dioxins in particular are very stable even at temperatures of 1000.degree. C. so that they do not decompose in a grate burning system. Dioxin contents have already been documented in flue ash and in cleaned gas dust of various garbage incinerating systems (see "Die Zeit", 2 Mar. 1984 "Dioxin, der Racher aus der Retorte").
The extreme toxicity of dioxins is known (e.g., "Bild der Wissenschaft", November 1984, pp. 64 through 76). Conventional garbage incineration in grate furnaces has thus been questioned in general (Chemical Engineering, June 6, 1983, pp. 20 through 64).
Theoretically, dioxins which are produced during combustion could be decomposed if the grate firing were to be followed by an after-burning stage having a firing temperature of at least 1200.degree. C. That, however, requires increased capital outlay as well as the use of conventional fuels and consequently would involve an extraordinary amount of expense.
Another possibility, that of performing the combustion by employing high temperature combustion air at higher firing temperatures, fails because 70 to 80% of the combustion air in grate firing is supplied under the grate as primary air and simultaneously serves the purpose of cooling the grate plates, so that heating is only possible within very narrow limits.
A heating of the secondary air would also cause only a relatively slight temperature increase of the flue gases, since the combustion temperatures below the secondary air admission would not be increased.
Finally, a significant increase in the combustion temperature in the grate furnace is not feasible for the reason that the slags present would melt and the grate plates would be attacked.
Due to the relatively low burning temperatures in the grate furnace, only a slight fraction of the heavy metals or heavy metal compounds of the refuse are volatilized. The remainder is carried out with the slag and this makes it difficult to utilize these slags for road construction and similar areas of employment.
Burning household waste in a rotary tubular kiln is currently being applied only to a slight extent. It is said that this procedure has several disadvantages. For example, the rotary kiln itself is more expensive than the grate furnace. The stoking effect, i.e., circulation of the combustible material in order to guarantee oxygen content, decreases with increasing diameter of the rotary tubular kiln so that only relatively small units, up to about 4 m diameter, have been utilized until now for household waste. Also, faultless ignition of the waste cannot always be guaranteed, particularly in the case of damp waste and waste low in calorific value so that support by ignition burners is required. This immediately raises the cost considerably.
In the case of a fluctuating garbage composition, there is a difficulty in insuring the carrying out of the sequence of the various process steps (drying, low temperature carbonization, ignition, burning) in defined regions of the kiln. Thus, there is a disadvantage that these process stages cannot be controlled independently of one another along the axis of the rotary tubular kiln.